Issue
152 March 2003
2-D
Optical Position Sensor
by
Roger Johnson & Chris Lentz
TYPICAL
APPLICATIONS
As continuous
position sensors, PSDs are unparalleled. Compared with
discrete element detectors, such as the charge-coupled
device (CCD) sensor, the PSD features nanometer positioning
resolution, sub-microsecond response times, simple interface
circuits, and high reliability.
Optical alignment,
involving a laser beam that’s used as a reference line,
is the most common application for a PSD. The PSD is
mounted on the system being tested (e.g., the wobbling
of a shaft, the straightness of machine tool axis, or
an aircraft fuselage that’s being assembled).
PSDs are also
used in more lethal applications. In heatseeking missile
systems, a PSD that’s sensitive to IR radiation is located
at the focal point of a lens that’s mounted behind a
clear-domed window on the front of the missile. After
the missile is launched, the outputs of the PSD drive
the missile’s fin actuators to keep the IR energy centered
on the PSD all the way to the target (i.e., the jet
exhaust of an aircraft).
The scanning
laser level is a more down-to-earth application. In
this instance, remote optical targets that contain a
1-D PSD pick up a spinning, level laser beam. These
systems are used for pouring concrete and installing
ceiling tiles in addition to laying pavement.
Note that
PSDs are also used in non-contact distance sensors,
which incorporate optical triangulation to measure the
range to a target surface. The sensor actively projects
a laser spot onto a surface. When the spot is viewed
at an angle different from that of the laser axis, the
imaged spot of light walks back and forth across a detector
as the distance to the surface changes.
POSITION
EQUATIONS
When light
falls on the PSD, the photocurrent collected by an electrode
is inversely proportional to the distance between the
incident position and the electrode. Several of the
equations in Figure 4 relate the photocurrents I1 and
I2 collected by the electrodes with position along the
detector, where L is the active length of the PSD and
I0 is the total photocurrent.
|
(Click
here to enlarge)
|
Figure
4—Study these equations carefully. In addition to
the position equations a through f, we’ve included
the equations for the following: g—the feedback
resistor; h and i—the optical power; and j and k—the
algorithm for the low-pass filter. |
In Figures
4a and b, you see the equations of a 1-D PSD for the
two photocurrents with respect to the center of the
detector. For these two equations, L is the active length
of the PSD, x is the position of the centroid of the
light falling on its surface, and I0 is proportional
to the incident power. Figure 5 illustrates the coordinates
used in describing the terms; the dot indicates the
centroid of a light spot.
|
|
| Figure
5—Take a look at the coordinates we’ve used to describe
the terms. Note the dot that indicates the centroid
of a light spot. |
The difference
of the two photocurrents is proportional to the position
and intensity of the centroid of light striking the
detector. As you can see in Figure 4c, dividing the
difference of the photocurrents by their sum cancels
the I0 term and yields a normalized position
value that’s independent of incident optical power.
Note that n is a dimensionless position value that ranges
from –1 to 1 (i.e., –1 £
n £ 1). You can solve for
x with the equation in Figure 4d.
The position
equations for a tetra-lateral/pincushion PSD are depicted
in Figures 4e and f. Note that x and y are the photocurrents
flowing from the PSD’s terminals. The denominator term
is proportional to the total incident power. Handily,
because the position value requires the sum term, the
PSD can easily sense optical power. The instantaneous
power is displayed with x and y position data.
